1. Field of the Invention
This present invention relates generally to a power conversion circuit and more particularly to a multiphase switching power converter.
2. Description of the Related Art
A typical power conversion circuit (e.g., a switching power converter) receives an input voltage and an input current and modifies the input voltage, the input current or both the input voltage and the input current to produce an output voltage and an output current. For example, a DC-to-DC converter receives input power from a DC voltage source at one voltage level and outputs a desired DC output voltage (typically, a regulated DC output voltage) at another level. A converter that includes a feedback loop to regulate one or more output parameters (e.g., voltage, current, etc.) is often referred to as a regulator. One embodiment of a converter is a switching converter that uses one or more switches to alternately connect and disconnect the voltage source to circuits that drive the output. The duty cycle of the switching is used to control the output voltage. The switching is typically controlled by a Pulse-Width Modulation (PWM) circuit.
The advancement of the microprocessor integrated circuit into the gigahertz frequency band of operation has led to the use of DC-to-DC converters that can operate in the multiphase mode. State-of-the-art processors are now operating with a core voltage ranging from 1.4 volts to 1.8 volts and with a core current in the range of 30 to 75 amperes. The continuous inductor current rating sets a typical limitation in output current that can be delivered by a single-phase converter. At normal operating frequencies, this current typically ranges between 2 and 20 amperes. Under these assumptions, a processor core needing 60 amperes requires a converter with four or more phases.
In a multi-phase switching converter, a PWM circuit provides a variable duty cycle signal to control the switching for each channel. The PWM signals are synchronous with different phases for each channel, thereby allowing each channel to be switched on at a different time. The multiple phases increase the output ripple frequency above the fundamental channel switching frequency and reduce the input ripple current, thereby significantly reducing the sizes of input capacitors and output capacitors, which are often large and expensive. Stress and heat on the components are also reduced because the output current is distributed among the multiple channels.
The DC current through each inductor is responsive to the duty cycle of its PWM signal and to the value of its voltage source. Each inductor has a current limit. Typically, more PWM circuits are used when more output current is desired. The output terminals of all the inductors from the PWM circuits are electrically connected to provide a single output of the power conversion circuit.
Since the output terminals of all the inductors are tied together, the conductors have substantially identical output voltages. The input terminal of each inductor has a rectangular wave voltage signal, which is derived by switching the input terminal between the input voltage source and ground. The duty cycles of the rectangular wave voltage signals of respective channels are affected by variations in the respective PWM circuits and switches (e.g., design tolerances, offsets, and timing variations). For example, a slight difference in the duty cycle can produce a significant difference in the DC current through the inductor in each channel.
High efficiency power conversion circuits typically use inductors with low core loss (e.g., ferrite inductors). When the peak design current is exceeded (i.e., saturation), the inductance of ferrite core material collapses abruptly which results in an abrupt increase in inductor ripple current and output voltage ripple. Thus, it is important to keep the inductor core from saturating.
Forced current sharing is used to cause all the channels to contribute substantially identical currents to the output. Forced current sharing prevents an inductor in one of the channels from saturating. Prior art systems sense the current in each channel and adjust the respective duty cycles to produce the same current for each channel. Current sensing decreases the efficiency of the power conversion circuit because power is dissipated by a sensing resistor. Further, current sensing requires an undesirable ripple voltage across the sensing resistor in order to work properly. Other prior art systems employ costly precision design and trimming in an attempt to achieve accurate current sharing without sensing resistors. Typically, phase current mismatches are on the order of 30 percent or greater when employing precision duty cycle matched converters, necessitating the use of significantly higher current MOSFETs and inductors.
In a typical multiphase converter, the frequency of each phase is identical and the phase relationship between the various phases is adjusted to produce phase symmetry in the context of 0 to 360 degrees for one cycle. The typical phase relationship is 360 degrees divided by the number of phases used (e.g., in a two-phase converter, the phases are 180 degrees apart, in a three-phase converter the phases are 120 degrees apart, etc.). This phasing is useful because the input ripple current and the output ripple current typically have maximum reduction when the phases are added together symmetrically. As in the output current, the ripple current is reduced by half and the ripple frequency is twice that of the operating frequency when two phases are operated in parallel. Thus, smaller input filter capacitors and smaller output filter capacitors may be used for a given design.
Another feature of a multiphase converter is the improvement of the load transient response of the converter with each additional phase. Typically, a PWM operates at a frequency around 500 kHz. Some converter designs are approaching a 1 MHz operating frequency to improve transient response. Some transient specifications are approaching 60 amperes per microsecond transient response. From the position of the load, looking back into the DC-to-DC converter, a two-phase 500 kHz converter looks substantially the same as a single 1 MHz converter. Therefore, a four-phase 500 kHz converter has approximately the performance as 2 MHz converter. In general, more phases added symmetrically will have the benefits of increased load current, improved transient response, better distribution of the heat loss, less input ripple current, less output ripple current, and potentially improved reliability.
Multiphase converters require the desired phase relationship to be maintained between the various outputs of the converter. Some converter systems use a reference/slave arrangement where multiple pins are used between integrated circuits (ICs) to set up the multiphase solution. In a reference/slave arrangement, one IC is the reference and the remaining IC's are the slaves. Slave ICs are coded to be placed in the proper phase relationship to the reference. In most cases, the ICs need a clock that runs at 4 to 8 times the reference clock frequency. The ICs include counters and decoders to produce the proper phasing from the clock. One exemplary system uses phase-lock loops between ICs to configure a multi-phase solution. Such ICs are very complex, and several pins are required for each IC to enable the IC to define a phase relationship with respect to the other ICs.